1. Field of the Invention
The present invention relates to an apparatus of a catalyst for purifying exhaust gas exhausted from an automobile and a method for purifying exhaust gas using this catalyst apparatus, and in particular, relates to an apparatus of a catalyst for purifying exhaust gas suitable as a three-way catalyst (TWC) apparatus to be used for purifying carbon monoxide (CO), a hydrocarbon (HC) and a nitrogen oxide (NOx) in exhaust gas exhausted from a gasoline automobile and a method for purifying exhaust gas.
2. Description of the Prior Art
Various catalysts have been used in response to objective thereof, in an apparatus of a catalyst for purifying exhaust gas exhausted from an internal combustion engine of an automobile or the like. As major catalyst components thereof, there are platinum group metals, which are usually used with being loaded, in a highly dispersed state, on a refractory inorganic oxide with high surface area, such as activated alumina (refer to Patent Document 1).
As for the platinum group metals as a catalyst component, platinum (Pt), palladium (Pd) and rhodium (Rh) are known, which have been used widely as the catalyst for purifying exhaust gas exhausted from an internal combustion engine of an automobile or the like. In the above TWC, catalytically active species superior in oxidation activity, such as Pt and Pd, and Rh superior in purification activity of NOx are used in combination in many cases. In recent years, strictness of regulations for harmful substances, in particular, NOx contained in exhaust gas has been increasing. Therefore, it is necessary to use efficiently Rh superior in purification activity of NOx. In addition, output of Rh is low, Rh is expensive and market price thereof has been rising in recent years. Therefore, it is preferable to reduce use amount of Rh as catalytically active species, in view of resource protection or a cost aspect.
In addition, in the catalyst for purifying exhaust gas, the addition of various co-catalyst components other than the platinum group metals in the catalyst for purifying exhaust gas have been studied, to attain enhancement of still more purification performance. As such co-catalyst components, there has been known an oxygen storage component (OSC), an alkaline earth metal, zirconium oxide, zeolite or the like.
Among them, the OSC is one for storing and discharging oxygen in exhaust gas, and cerium oxide has been known. Cerium oxide stores oxygen as CeO2, when oxygen concentration in exhaust gas is high, and discharges oxygen by being converted to Ce2O3, when oxygen concentration is low. Oxygen discharged is active oxygen and promotes purification of HC or CO by being utilized in oxidation action by Pt or Pd. In addition, by storage and discharge of oxygen, the OSC also acts to buffer the oxygen concentration change in exhaust gas. By this action, in the TWC, purification performance of exhaust gas enhances. The TWC performs oxidation and reduction by one catalyst, and has a range of exhaust gas components suitable for purification in view of designing. This range depends on air/fuel ratio in many cases. Such a range is referred to as a window, and in many cases, exhaust gas combusted at the vicinity of theoretical air/fuel ratio is set as the window region. By buffering variation of oxygen concentration in exhaust gas, this window region can be maintained for a long period, and purification of exhaust gas is performed efficiently. It has been said that this has influence, in particular, on purification performance of NOx by RH.
As such cerium oxide, pure cerium oxide may also be used, however, it is used as a composite oxide with zirconium (refer to Patent Document 2). The cerium-zirconium composite oxide has high heat resistance, and is said to have high storing and discharging rate of oxygen. Reason for this is considered that a crystal structure of the cerium-zirconium composite oxide is stable, by which an action of a cerium oxide, which is a major OSC component, is not obstructed, and can be utilized in an action as the OSC up to the inside of particles.
In such purification of NOx by Rh, it is considered for a steam reforming reaction to be promoted as follows via the Rh component.HC+H2O . . . →COx+H2  (1)H2+NOx . . . →N2+H2O  (2)
And, zirconium oxide is said to promote the steam reforming reaction by using together with the Rh component (refer to Patent Document 3).
As a co-catalyst component, other than this, the alkaline earth metal such as a Ba component has been known (refer to Patent Document 4). The Ba component temporarily stores NOx contained in exhaust gas, and purifies NOx stored by reducing to N2 by a reducing component contained in exhaust gas.
In general, NOx generates in a large quantity, when fuel supplied to an engine is less and amount of air is much. The Ba component temporary absorbs NOx generating in this way.
NOx absorbed by the Ba component is discharged from the Ba component when concentration of NOx in exhaust gas is low, and CO concentration becomes high. In continuation of the above example, this is derived from a reaction of the Ba (NO3)2 with CO to become BaCO3 and may be said chemical equilibrium. NOx discharged from the Ba component, as described above, reacts with reducing components at the surface of the Rh component to be reductively purified.
Such a co-catalyst component may be used in combination of two or more kinds, for example, the TWC using the Ba component and cerium oxide has been known (refer to Patent Document 5). However, purification performance may be decreased depending on a combination of catalyst materials, for example, presence of the Rh component and the Ba component in the same composition is reported to decrease purification performance of NOx (refer to Patent Document 6). Reason for this is not certain, however, because the alkaline earth metal component has an action of storing NOx, it is considered that purification performance of NOx in the Rh component is obstructed, or the Ba component and the Rh component may produce an alloy.
In this way, there are various combinations of catalyst components, and because of passing a complicated reaction route by mutual interaction of the catalyst components, combinations of the catalyst components exerting purification performance most have been searched by comprehensive study thereof.
It should be noted that, the catalyst for purifying exhaust gas may be enough to be arranged at one place in a flow passage of exhaust gas, however, there may be the case where two or more are arranged. Because in this way, surface area of the catalyst is increased, purification performance of exhaust gas is enhanced. However, as described above, because the catalyst for purifying exhaust gas such as the TWC has a window region in view of designing, simple arrangement of a plurality of catalysts may not provide desired catalyst for purifying exhaust gas in some cases. It is because components of exhaust gas passed through the catalyst at the front stage are different from those of exhaust gas just after being exhausted from an engine, it is necessary to design composition of exhaust gas thus changed as the window region.
The present applicant has also proposed a catalyst system, where two catalysts having a platinum group metal and an oxygen storage component (OSC) as the catalyst components are arranged in a exhaust gas flow passage (refer to Patent Document 7). In this way, purification performance of exhaust gas not attained conventionally was able to attain.
However, in recent years, regulations for exhaust gas have been increasingly strict, and appearance of a catalyst apparatus is desired which exerts more superior exhaust gas purification performance using a plurality of catalysts. Regulation values for, in particular, NOx, among exhaust gas, have become strict, and also for the TWC, there has been increasing necessity of an apparatus of catalyst for purifying exhaust gas superior in purification performance of NOx.
PRIOR DOCUMENTSPATENT DOCUMENTSPatent Document 1:JP-A-5-237390Patent Document 2:JP-B-6-75675Patent Document 3:JP-A1-2000/027508, page 14Patent Document 4:JP-A-2007-319768, paragraph 0003Patent Document 5:JP-A-03-106446Patent Document 6:JP-A-2002-326033, paragraph 0013Patent Document 7:JP-A-2008-68225